Filter Optical Flow for outliers

[shadow1runner]

@inproceedings{zingg2010mav, title={MAV navigation through indoor corridors using optical flow}, author={Zingg, Simon and Scaramuzza, Davide and Weiss, Stephan and Siegwart, Roland}, booktitle={Robotics and Automation (ICRA), 2010 IEEE International Conference on}, pages={3361--3368}, year={2010}, organization={IEEE} }

used a two-step filtering:

A simple threshold filter removes too large optical flow amplitudes. In a further step, an angular criterion is checked. Optical flow has to be tangential to a circle with its center at the center of the image with a deviation of up to 50°. This threshold seems to be large, it has been chosen experimentally and proved to work though. It is necessary, since the optical flow has not an exact circular shape. If the threshold is chosen smaller, optical flow close to the direction of travel might be filtered out, even if it was not wrongly matched. A remarkable amount of wrong matches could be extracted using these two criteria.

Todo:

• Is filtering necessary with a dense optical flow field?
• Analyze OF with and without filtering
• how to they determine the center of their circle? voting?

[shadow1runner]

Furthermore, the filtered output should return 0 for the OF vectors if the OF vectors were matched in a darkened area of the (fisheye) image. This implies to make a blob search starting at all four corners and ANDing the OF result vector with the logical OR of both masks.

A mask not necesserily needs to be rectangular (even though a Mat is, of course), see here for more information